Molecular Characterization of Auxin Efflux Carrier- ABCB1 in hexaploid wheat.

Auxin is an important phytohormone that regulates response, differentiation, and development of plant cell, tissue, and organs. Along with its local production, long-distance transport coordinated by the efflux/influx membrane transporters is instrumental in plant development and architecture. In the present study, we cloned and characterized a wheat (Triticum aestivum) auxin efflux carrier ABCB1. The TaABCB1 was physically localized to the proximal 15% of the short arm of wheat homoeologous group 7 chromosomes. Size of the Chinese spring (CS) homoeologs genomic copies ranged from 5.3-6.2 kb with the 7A copy being the largest due to novel insertions in its third intron. The three homoeologous copies share 95-97% sequence similarity at the nucleotide, 98-99% amino acid, and overall Q-score of 0.98 at 3-D structure level. Though detected in all analyzed tissues, TaABCB1 predominantly expressed in the meristematic tissues likely due to the presence of meristem-specific activation regulatory element identified in the promoter region. RNAi plants of TaABCB1 gene resulted in reduced plant height and increased seed width. Promoter analysis revealed several responsive elements detected in the promoter region including that for different hormones as auxin, gibberellic acid, jasmonic acid and abscisic acid, light, and circadian regulated elements.

An Ethylmethane Sulfonate Mutant Resource in Pre-Green Revolution Hexaploid Wheat.

Mutagenesis is a powerful tool used for studying gene function as well as for crop improvement. It is regaining popularity because of the development of effective and cost efficient methods for high-throughput mutation detection. Selection for semi-dwarf phenotype during green revolution has reduced genetic diversity including that for agronomically desirable traits. Most of the available mutant populations in wheat (Triticum aestivum L.) were developed in post-green revolution cultivars. Besides the identification and isolation of agronomically important alleles in the mutant population of pre-green revolution cultivar, this population can be a vital resource for expanding the genetic diversity for wheat breeding. Here we report an Ethylmethane Sulfonate (EMS) generated mutant population consisting of 4,180 unique mutant plants in a pre-green revolution spring wheat cultivar 'Indian'. Released in early 1900s, 'Indian' is devoid of any known height-reducing mutations. Unique mutations were captured by proceeding with single M2 seed from each of the 4,180 M1 plants. Mutants for various phenotypic traits were identified by detailed phenotyping for altered morphological and agronomic traits on M2 plants in the greenhouse and M3 plants in the field. Of the 86 identified mutants, 75 (87%) were phenotypically stable at the M4 generation. Among the observed phenotypes, variation in plant height was the most frequent followed by the leaf morphology. Several mutant phenotypes including looped peduncle, crooked plant morphology, 'gritty' coleoptiles, looped lower internodes, and burnt leaf tips are not reported in other plant species. Considering the extent and diversity of the observed mutant phenotypes, this population appears to be a useful resource for the forward and reverse genetic studies. This resource is available to the scientific community.

Comparative analysis of ABCB1 reveals novel structural and functional conservation between monocots and dicots.

Phytohormone auxin plays a critical role in modulating plant architecture by creating a gradient regulated via its transporters such as ATP-binding cassette (ABC) B1. Except for Arabidopsis and maize, where it was shown to interrupt auxin transport, ABCB1's presence, structure and function in crop species is not known. Here we describe the structural and putative functional organization of ABCB1 among monocots relative to that of dicots. Identified from various plant species following specific and stringent criteria, ZmABCB1's "true" orthologs sequence identity ranged from 56-90% at the DNA and 75-91% at the predicted amino acid (aa) level. Relative to ZmABCB1, the size of genomic copies ranged from -27 to +1.5% and aa from -7.7 to +0.6%. With the average gene size being similar (5.8 kb in monocots and 5.7 kb in dicots), dicots have about triple the number of introns with an average size of 194 bp (total 1743 bp) compared to 556 bp (total 1667 bp) in monocots. The intron-exon junctions across species were however conserved. N-termini of the predicted proteins were highly variable: in monocots due to mismatches and small deletions of 1-13 aa compared to large, species-specific deletions of up to 77 aa in dicots. The species-, family- and group- specific conserved motifs were identified in the N-terminus and linker region of protein, possibly responsible for the specific functions. The near-identical conserved motifs of Nucleotide Binding Domains (NBDs) in two halves of the protein showed subtle aa changes possibly favoring ATP binding to the N-terminus. Predicted 3-D protein structures showed remarkable similarity with each other and for the residues involved in auxin binding.